1. Field of Invention
The present invention relates to a circuit board, and more particularly to a circuit board with plating bar.
2. Related Art
Accompanying to the development of corresponding technologies of electronic industry and the shrinkage in dimensions of the product, the IC designer and the IC manufacturers face to many challenges in process such a compact product. One of those challenges is more pads and conductive traces formed on the surface of the circuit board for signal or power transmission. In the conventional circuit board, a nickel/gold (Ni/Au) layer is generally covered on the surface of the pad. Thus, the golden wire and the pad of the circuit board are easily joined together during the wire bonding process. Similarly, the soldering pad, which is typically made of copper, of the packaging substrate is covered by the Ni/Au layer to prevent against oxidization and to improve the quality of electrical and mechanical connection of the solder balls.
The electroplating process is to form a metallic layer by electrolysis. A metal sheet for plating conductive layer acts as an anode, the electrolyte is an ion solution containing metallic ions, and the plated object acts as a cathode. The metallic ions in the electrolyte are attracted and moved to cathode after applying voltages to anode and cathode. The metallic ions are then plated on the plated object after reduction. In order to form the conductive layer, a plurality of plating bars are needed to be disposed on the circuit board for electrically connecting the pads with cathode through the plating bars.
As shown in FIGS. 1a and 1b, a conventional circuit board 100 has a plurality of pads 111 acting as electrical connections of the circuit board 100 and a device (not shown). The pad 111 is a wire bonding pad or a soldering pad, which may connect to another device on the same surface through a transmitting trace 112, and may also connect to a conductive via 150 through the transmitting trace 112 for connecting with an internal circuit of the circuit board 100.
As shown in FIG. 1b, the circuit board 100 includes four conductive layers and three dielectric layers 101, 102 and 103. The conductive layers are respectively isolated by one dielectric layer. In the prior art, the top conductive layer and the bottom conductive layer of the circuit board 100 are generally used for disposing signal layout and the pads 111. The intermediate conductive layers 120 and 130 act as the reference planes, for example the conductive layer 120 is a power plane and the conductive layer 130 is a ground plane. The circuit board 100 has a through hole 105. A conductive via 150 is formed by disposing a conductive material on side wall of the through hole 105 for electrically connecting between different conductive layers. The transmitting trace 112 is for example connected to a transmitting trace 141 on the other side of the circuit board 100 through the conductive via 150. A signal is thus transmitted to another device (not shown) through the signal transmitting structure including the pad 111, the transmitting trace 112, the conductive via 150 and the transmitting trace 141.
The pad 111 includes at least one electroplating metallic layer such as Ni/Au layer is formed by a plating process. In order to electrically connect the pad 111 to a plating electrode during the plating process, the signal transmitting path is connected to a plating bar 160. In FIG. 1a, the plating bar 160 is connected to the transmitting trace 112 or the conductive via 150. The plating bar 160 may be located at another conductive layer different from that of the pad 111. In FIG. 1b, a plating bar 160 on the power plane 120 is connected to a pad 111 through the conductive via 150. In addition, the plating bar 160 will also extend to an edge 104 of the circuit board 100.
However, the layout for a plurality of plating bars will occupy the available area for circuit layout. The signal transmitting path needs not to pass the plating bar in the signal transmitting structure, that is, the signal transmitting structure has an open stub. The open stub effect of redundant plating bars causes the impedance of the signal transmitting path is not matched during the application of high frequency, causes a noise during the signal transmission, and lowers signal transmitting quality.
In spite of other solutions have been disclosed in the prior art, which include forming the conductive layers without plating or removing the plating bars in the following process. However, these solutions cause the limitation to circuit design and the increasing to production cost. It is therefore an important subject of the present invention to provide a circuit board to reliably and effectively reduce the influence of the plating bar to the signal transmitting structure.